I have a G-Raid w/Thunderbolt and out of the box according to the black magic app I got a transfer read / write rate of 327 MB/s. I reformatted the drives to allow for a 64k/block size and added 2.5 TB of data. Now the black magic app reports at most 297 MB/s read / write speeds. Any idea of why the slow down. Is this expected behaviors? Using OS X 10.9.2 with a rMBP late 2013.

Thanks for posting. What capacity G-Raid do you have? 4TB or 8TB? If you have the 4TB model you have passed the 50% capacity point and you usually do see a bit of a drop in performance after passing that point.